This invention relates to a nuclear fuel pellet, and in particular nuclear fuel pellet (hereinafter referred to merely as pellets) received into a clad casing equipped with sealing and plugs.
Generally, a nuclear fuel rod for use in an atomic reactor of an atomic power plant has a construction shown in FIG. 1. In FIG. 1, a plurality of pellets 2 are received into a clad casing 1 and the casing is sealed by end plugs 3 and 4. The pellet 2 is formed by compression molding powdered ceramic nuclear fuel such as uranium dioxide and a mixture of uranium dioxide and plutonium dioxide into a cylindrical mass and sintering it at high temperature. A spring 5 is disposed in a plenum chamber 6 between the pellets and one end plug to prevent displacement of pellets 2. A helium gas is filled in the plenum chamber 6 as well as between the pellets 2 and the casing 1.
When an atomic reactor using such pellets 2 is operated, the pellet 2 generates a heat by a nuclear fission and becomes a high temperature. Since at this time a ceramic pellet has a small thermal conductivity, the central portion of the pellet becomes a very high temperature in comparison with the peripheral portion of the pellet. In this way, a temperature distribution is produced in the pellet 2. Such a thermal expansion difference causes, as shown in FIG. 2, the end surfaces 2a and 2b of the pellet to be deformed in a convex way and the side surfaces 3c of the pellet to be deformed in a concave way. As a result, the pellet is deformed in the form of an hour glass. The radial deformation of the pellet is greater at the end edge portions than at the central portion due to an "end surface effect".
In this way, the pellet 2 becomes a high temperature and is greatly expanded into contact with the casing 1 with the result that a mechanical interaction (i.e. PCMI) occurs between the pellet 2 and the casing. The nuclear fuel rod is designed such that, in order for the casing not to be greatly deformed under the condition of PCMI, an axial expansion of the pellet 2 is absorbed by the plenum chamber 6 and a radial expansion is absorbed by the clearance 7. The pellet under the usual intra-reactor conditions is not always centrally located in the clad casing and the narrow clearance 7 is often left to one side. In consequence, it is considered that a weak PCMI is always present there. In order to reduce PCMI to a maximum possible extent, pellets of varying shapes have been proposed as will be explained below.
The pellet 10 shown in FIG. 3 has plate-like dimples 11 and 12 which are formed one at each of the end surfaces 8 and 9. The pellet 13 shown in FIG. 4 have its end edge portions chamfered to provide bevel portions 14 and 15, respectively. The pellet 16 shown in FIG. 5 has a central through bore 17. These pellets 10, 13 and 16 are all designed to absorb some deformation and alleviate PCMI, but not sufficient since they have advantages and disadvantages. Stated briefly, the pellet 10 shown in FIG. 3 absorbs an axial expansion, but a radial expansion is increased. A force from an overlying pellet is concentratedly applied to the marginal edge portion of the end 8 of the pellet 10 to cause further deformation to occur. The pellet 13 shown in FIG. 4 absorbs a radial expansion at the end portions, but no axial deformation is absorbed. Furthermore, no uniform compression is effected during the manufacture of the pellet, developing cracks called "capping". The pellet 16 shown in FIG. 5 is not fitted for mass production, since a complicated process is required in compression molding the poweder mass. Moreover, during the burning of the pellet within the atomic reactor fragments produced by the breakage of the pellet 16 and dropped through the through hole 17 and accumulated on the bottom of the casing 1, thereby making a temperature distribution nonuniform.